When teams win, not only do established fans take part in the team (ticket sales, jersey sales), but the team gains new fans.
Living out in California now, I've met a number of Bills fan that were not born in Buffalo or Western New York area, but became fans of the Bills during the 90's Super Bowl run.
Now, with the NFL expanding in popularity and exposure (more TV exposure, Directv, Sirius radio, etc.), there is a new generation of fans exposed to the NFL, and looking for a team to root for.
Unfortunately, this is not the Bills. We have limited nation exposure not just because of the small market, but because the team has been a non-playoff contender for a decade.
And this is one of the lesser-discussed side-effects from having a team that continues to lose and miss the playoffs.